Abstract
In this research, we studied the creep properties of a selective laser melting (SLM)-processed γ′-strengthened IN939 superalloy along the building direction compared to a conventional cast alloy as a reference specimen. In the as-built condition, high-density dislocations were formed as a result of the SLM process due to the generation of the larger thermal gradient. Post-heat treatment was necessary to obtain specific mechanical properties to match industrial requirements. Two heat treatment conditions were used: the first was lower temperature heat treatment (LTH: solution treatment at 1160 °C/4 h + aging at 850 °C/16 h). The second was higher temperature heat treatment (HTH: solution treatment at 1240 °C/6 h + aging at 850 °C/16 h). Creep tests were conducted at 816 °C/250 MPa. The first and second heat treatment conditions were used for the SLM specimens, but only the first condition was used for the cast alloy (cast-LTH). The SLM specimens in the as-built and LTH conditions showed very poor creep life but good elongation. The poor creep life of the as-built specimen was caused by high dislocation density and the small recrystallized grains formed during testing. In the LTH specimen, poor creep life was due to the formation of the undesirable η phase at the grain boundary, as well as the formation of small recrystallized grains during testing. The creep life of the HTH specimen was 2.7 times longer compared to the LTH specimen. This was because these specimens were covered with recrystallized grains that included low-density dislocations, columnar grain morphology with random orientation, improvement in γ′ precipitate size, and elimination of undesirable η phase. The cast LTH specimen showed longer creep life than SLM specimens because of coarser grains with low-density dislocations, γ′ precipitate coarsening during the creep, and the presence of carbides at grain boundaries. In addition, the cast LTH specimen exhibited lower creep strain rate than SLM specimens also helped in creep life improvement.
Highlights
Ni-based superalloys are widely used in aircraft engines and power generation industries because of their excellent properties like tensile, creep, fatigue, corrosion, and oxidation resistance at extremely higher temperatures (~850 ◦ C) [1]
The selective laser melting (SLM) IN939 block was separated from the base plate and sliced into thin plates (3.1 mm thick) by using an electro-discharge wire cutting machine (Brother HS-300, Tokyo, Japan) and creep specimens were cut from the plates
Inverse pole figure (IPF) micrographs of the as-built specimen showed columnar grains with a preferential alignment of along the building direction (Figure 3a). This is because of epitaxial grain growth during the SLM process, and it leads to strong bonding between the layers [28]
Summary
Ni-based superalloys are widely used in aircraft engines and power generation industries because of their excellent properties like tensile, creep, fatigue, corrosion, and oxidation resistance at extremely higher temperatures (~850 ◦ C) [1]. To obtain these properties at higher temperatures material should survive without failure for a certain period. The conventional cast process is complex and cost-oriented to make intricate shapes like turbine blades in aerospace industries. This process results in macro segregation due to melt-related problems such as porosity and segregation, which degrade mechanical properties [2]. The only way to mitigate this problem is through rapid solidification of metal or use of a solid-state sintering process or additive manufacturing process
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